Layered rhodium and nickel

ABSTRACT

The invention concerns a catalyst in which a nickel component is deposited on an inert support, followed by an alumina coating and a rhodium component. The catalyst is particularly useful for removing nitrogen oxides from automotive exhaust.

United States Patent 1191 Barker 1 Aug. 5, 1975 LAYERED RHODIUM ANDNICKEL [75] Inventor: George E. Barker, St. Louis, Mo.

[73] Assignee: Monsanto Company, St Louis, Mo.

[22] Filed: June 8, 1973 [21] Appl. No.: 368,546

Related US. Application Data [63] Continuation-in-part of Ser. No.330,531, Feb. 8,

[52] US. Cl. 252/455 R; 252/466 B; 423/2135 [51] Int. Cl. B01j 11/08;801 ll/22 [58] Field of Search 252/466 B, 455 R;

[56] References Cited UNITED STATES PATENTS 3,230,034 H1966 Stiles423/2132 3,410,651 ll/1968 Brandenburg et al. 423/2132 3,433,581 3/1969Stephens et al 423/2135 3,554,929 l/1971 Aarons 252/462 3,809,743 5/1974Unlarld et a1. 423/2135 Primary ExaminerW. J. Shine [57] ABSTRACT 13Claims, No Drawings LAYERED RHODIUM AND NICKEL This application is acontinuation-in-part of my copending application Ser. No. 330,531, filedFeb. 8, 1973.

BACKGROUND OF THE INVENTION The present invention relates to means ofremoving noxious materials from engine exhaust and to a dual functionalrhodium and nickel catalyst suitable for such purpose. In particular,the invention concerns processes for reducing nitrogen oxides present inexhaust of automotive engines and the use of the addition of air inconjunction with use of rhodium and nickel catalyst of layered structurein such process.

It is well known that combustion is incomplete when hydrocarbons areburned in combustion engines, whether of the internal combustion type orother alternative vehicular power sources. Consequently automotiveexhaust contains carbon monoxide and hydrocarbons along with otherproducts of incomplete combustion with are generally considered to benoxious. Among the pollutants formed are nitrogen oxides, termed NO (NOand N The NO, emissions will vary with driving modes, but do not oftenexceed 0.3 or 0.4% by volume of the exhaust gases, and may averagearound 0. However, NO, is definitely classed as a pollutant and mucheffort has been spent on methods of removing NO, from exhaust gases.

Noble and other metal catalysts have previously been proposed for use inremoving N0 A problem frequently encountered with such catalysts is theproduction of ammonia. Ammonia is definitely an undesirable productsince it is subject to re-oxidation to NO, under oxidizing conditions,and such conditions are definitely planned for an oxidation catalystconverter which will oxidize the hydrocarbon and other components of anexhaust gas stream.

The proposals for catalytic removal of NO, generally involve reductionof the N0 and therefore employ reducing conditions. Reducing conditionscan be obtained by control of the engine air-to-fuel ratio, i.e. byemploying a rich air-to-fuel ratio. Such an air-to-fuel ratio producesan exhaust with a relatively high content of oxidizable componentscompared to the small amount of oxygen present in the exhaust. Thereforea rich air-to-fiiel ratio is appropriate for reduction of NO,. However,with many catalysts it has been found that a rich air-to-fuel ratioleads to excessive ammonia formation. The ammonia formation is affectedby the ratio of the oxidized components of the exhaust gas and theeffects of a rich air-to-fuel ratio on ammonia formation can bemitigated by the addition of air when a rhodium catalyst is used.

It has previously been recognized that temperature has an effect uponthe amount of ammonia formation over various noble metal and othercatalysts. In particular it has been recognized that ammonia formationwith nickel and similar catalysts is much greater at temperatures around500C than it is at somewhat higher temperatures. It has therefore beenproposed to add air to the exhaust gases so that the resulting oxidationwill produce higher temperatures and make the catalytic removal of NO,more effective with respect to avoidance of ammonia formation. Thepractice for implementing this procedure has been to add the air in theengine manifold. Equipment adaptable for such addition is already onproduction model vehicles and addition at this location is consistentwith the intention of having a reaction exotherm to raise thetemperature in the following NO, converter. This method of air additionis apparently very satisfactory for its intended purpose of raising thetemperature. However the air addition as described herein for a rhodiumand nickel layered catalyst is for purposes beyond that of raising thetemperature. Nickel catalysts are among those which have been proposedfor removal of NO and air addition has been used in conjunction withsuch catalysts to obtain high temperatures. It has also been recognizedthat various noble metals can be included along with nickel on a supportto serve as promoters.

SUMMARY OF THE INVENTION The present invention concerns a dualfunctional rhodium and nickel catalyst in which the rhodium and nickelconstitute layers on a support. The catalyst is formed by depositingnickel on an inert support, coating the nickel with a high surface areacatalyst support material upon which catalytic metals can be dispersed,i.e. alumina, and depositing rhodium on the catalyst support material.The invention is also concerned with use of the described catalyst toremove NO, from automobile exhaust, with advantages being especiallyevident is useage over a varying temperature range, and with use of richcarburetion in conjunction with addition of supplemental air to lessenammonia formation. A catalyst of the described structure is moreresistant to physical degradation upon aging that are similar catalystsin which the nickel is deposited upon an activated alumina coating.

DETAILED DESCRIPTION OF THE INVENTION The present catalysts involve astructure in which nickel is deposited or coated on an inert support andrhodium is dispersed on an alumina coating. The structure can be formedby depositing nickel on an inert support such as a cordierite monolith,then depositing an alumina coating on the nickel, and next depositingrhodium on the alumina. The individual deposition procedures can becarried out in customary manner. The nickel is ordinarily deposited froma solution of nickel salts, followed by drying and calcining. Thealumina is ordinarily deposited from a dispersion of alumina powder,followed by drying and calcining. The rhodium is similarly depositedfrom a solution of rhodium salts, followed by drying and, optionally,calcining. The present catalyst is designed to have the effect of bothrhodium and nickel as active components. Nickel is used in fairly highamounts and does not need a support of high surface activity, such asalumina, in order to be well-dispersed and effective. In fact aluminatends to make nickel depositions less stable toward physical degradationupon temperature or other environmental changes, or otherwise, interactswith the nickel and cause poor properties. Therefore the nickel isdeposited on an inert surface, such as that of a bare monolith material.By contrast, rhodium is used in very low amounts and must be highlydispersed on a surface of high activity, such as an alumina of highsurface area. Therefore an alumina surface is provided prior todeposition of the rhodium. The alumina is therefore deposited on thecarrier after the nickel. The nickel, of course, must be available tocontact the exhaust gases, so it is apparent that the alumina coating isnot a complete coating impervious to gas. The rhodium is then welldispersed upon the alumina surface. In order to obtain the desiredeffect from very small concentrations of rhodium, it is preferred thatthe rhodium be well dispersed on the surface of the alumina without muchpenetration into the alumina. The catalyst can,

for simplicity, be viewed as a layered structure in which a nickelcoating is present on an inert support, with an alumina layer over thenickel layer, and a rhodium layer over the alumina. However, it will berecognized that irregular, porous materials are involved and that theimpregnation procedures involve penetration of materials into thesurface. The monolithic structures used have irregular surfaces, and thealumina coatings formed are characteristically of very high surfacearea. Moreover the amounts of materials and methods of application arenot ordinarily such as to produce complete, continuous coatings, but thecoatings may have voids and irregularities. Thus the catalysts arereferred to herein as layered catalysts only to characterize thestructure in a broad sense. The main factor is that the nickel is fixedto a support other than the alumina upon which the rhodium is dispersed.There are advantages in having the nickel dispersed upon an inertsupport rather than upon alumina.

A variety of procedures can be employed in applying the differentcomponents of the present catalysts. Generally the nickel and rhodiumcomponents are added by immersion or by spraying appropriate solutionsof the compounds. For example, rhodium compounds which can be usedinclude the nitrate, halide, acetonulacetonate, or an appropriaterhodium carbonyl. An appropriate solvent is chosen, depending upon thecompound chosen. Thus water solutions of Rh(NO suffice, while benzenemay be chosen for Rh (CO) With impregnation procedures, minimum solutiontechniques have certain advantages, particularly with the low levels ofrhodium utilized herein. Thus, if excess solution techniques areemployed with aqueous solution of Rh (NO;;);; or RhCl selectiveabsorption can occur resulting in less control as to the amount ofactive deposited. The use of excess solution techniques, for example inadding rhodium to an alumina coated monolith, however, can be utilizedso long as care is taken to account for selective absorption. Thisgenerally can be accomplished simply by control of solutionconcentration and absorption time.

Alumina coatings can be applied in a variety of ways. A suitableprocedure involves use of dispersions of alumina poweder, such as can bemade conveniently by agitation of alumina powder in aqueous acid. Thebody to be coated is then dipped in the suspension. When monolithicsupports are to be coated it is advantageous to clear the channels ofexcess solution after dipping, for example by passing a suitable gasthrough the monolith or by other suitable means. After drying and aircalcining at about 500C, adhering coatings of an appropriate thicknessfor use in the present invention result.

600C; then immersion in an alumina dispersion, drying at l -200C,calcining in air at 400 to 600C; then immersion in a rhodium nitratesolution and drying at 1 10 to 200C. The catalyst can then be used assuch,

with decomposition of the rhodium nitrate salts if utilized atsufficiently elevated temperatures, such as encountered in treatment ofautomotive exhaust gases. If desired, the catalyst can be calcined priorto use, e.g. at 400 to 600C, in air, or, optionally in a reducingatmosphere.

The exhaust treatment procedures of the present invention are effectivewith rhodium catalyst containing various concentration levels ofrhodium. However, because of cost and availability considerations itwill generally be desirable to use low concentrations, and there willseldom be any reason for the rhodium exceeding 1% by weight of thesupport, or even 0.1% by weight thereof. In fact, the rhodium issurprisingly effective at extremely low loadings, substantially lessthan 0.01% by weight, such as less than 0.005% by weight, or 0.003% oreven 0.001% or less by weight of the support. While the proceduresherein can employ various rhodium concentrations, the fact that goodresults can be achieved with very low rhodium loadings is especiallysignificant. The amount of rhodium will generally be in the range of0.0005 to 0.05 weight parts per weight parts support. The amounts ofnickel will generally be in the range of 0.5 to 50 weight parts per 100weight parts of inert support or more narrowly l to 20 weight parts.Five or more weight parts nickel are often used. Amounts of alumina canvary widely but are often in the range of 0.1 or 0.2 to 20 weight partssupport. However, there is usually no need to use large amounts ofalumina in view of the very small amounts of rhodium to be dispersed,and therefore amounts in the range of 2 to 6 weight parts may bepreferable. There may be advantage in keeping the amount of aluminarelatively low in order to enhance accessibility of the nickel componentof the catalyst, such as having at least as much nickel as alumina on aweight basis or having the amount of nickel at least twice that of thealumina on a weight basis. The parts herein are with reference torhodium and nickel as metal, although the materials may be in otherforms during various conditions of use. The rhodium and nickel arepresumably in elemental form when employed under reducing conditions intreatment of automotive exhaust. However they may also exist at times invarious oxidized states, such as rhodium or nickel oxides, or variousother compounds, and the catalysts are considered within the inventionregardless of the particular state of the catalytically active rhodiumand nickel components.

The catalysts of the present invention can be utilized for variouspurposes, including various oxidation and reduction reactions. Howeverthey are particularly adapted for the treatment of exhaust fromcombustion engines by contact at high temperatures to remove pollutantstherefrom such as hydrocarbons, carbon monoxide, and especially nitrogenoxides. Rhodium catalysts without a nickel component can be utilized forthis purpose but suffer some loss in effectiveness in particular hightemperature ranges such as ranges upwards of 700 or 750C inlettemperatures to catalytic converters. Such temperatures can occur insome phases of engine operation. The present combination catalyst hasimproved effectiveness at such high temperatures.

The catalysts described herein are particularly suited for removal of NQin a procedure in which an engine is operated with a rich air-to-fuelratio range, and supplemental air is added to the exhaust gas streamprior to contact with the catalyst. The amount of air preferablyapproaches the maximum which can be added without making the conditionsoxidizing in the air-to-fuel ratio operating range. One of the objectsof the aforesaid addition of supplemental air is to widen the range ofair-to-fuel ratios over which specified levels of NO control can beattained and to improve the degree of NO, control. In view of theresults attributed to the supplemental air, it can conveniently betermed bootstrap air and that term is at times used herein to designateair added to the exhaust stream in advance of the NO, converter. A modeof operation involving a rich air-to-fuel ratio and bootstrap air willgenerally be referred herein as a bootstrap operation or mode ofoperation.

The effectiveness of a system in removing NO considering both NOreduction and ammonia production, can be summarized by taking, on amolecular basis, the sum of NO and NH remaining, as a percentage of theoriginal NO present. For convenience this percentage is sometimesdesignated herein by the Greek letter Omega Q. The bootstrap mode ofoperation with rhodium containing catalyst causes 9. to remain at lowlevels over a large range of air-to-fuel ratios.

Air-to-fuel ratios have a significant bearing on the efficiency of theoperation of an automobile engine, and also on the composition of theresulting exhaust gases. Such ratios can be reported in terms ofpercentages of air with respect to stoichiometric, but are commonlyreferred to as an A/F ratio and reported on a weight basis, e.g. poundsof air per pound of fuel, with the relationship of the A/F ratio to thestoichiometric point being particularly significant. The stoichiometricpoint may vary somewhat, but is generally around 14.7 pounds air perpound of fuel for gasolines used in automobiles. The A/F ratio can bereported as a AA/F, indicating by how many A/F units it differs fromstoichiometric, with units on the rich side of stoichiometric beingreported as AA/F. An automobile engine has to be adaptable to variousdriving conditions in the normal course of operations; because of thisand other factors, carburetors are ordinarily set to operate over aspecified range of AA/F ratios. Presently proposed carburetion controlmay involve an A/F carburetion range which is one AA/F unit wide, butmore precise carburetion controls may make it feasible to operate over arange which is only 0.5 A/F unit wide, or some other more limited range.The presently described bootstrap operation makes it possible to improveresults, i.e. lower 0, particularly at the rich side of an operatingrange, and is therefore useful with the kind of carburetion contemplatedfor automobile engines. The bootstrap mode of operation can be used withany of the rhodium catalysts described herein.

The addition of bootstrap air is useful because ammonia productionvaries with the ratio of oxidizable components to oxygen. It happensthat the ratio of CO to H in automotive exhaust is ordinarily fairlyconstant, e.g. usually in the range of about 3.0 to 3.5 and quite oftenabout3.2. Therefore the relationships involved between oxidizablecomponents and oxygen can conveniently be stated in terms of CO/O ratio.The desirable CO/O ratios in a bootstrap operation will depend upon theintended levels, and may vary with the particular rhodium/nickelcatalyst or other factors, but will usually be in the range of 1.5 to 5on a molecular basis or more preferably 1.5 to 3 or 4. Even so, the useof bootstrap operation ordinarily gives some improvement in results ifthe. CO/O ratio ranges as high as 10 or 20 or more in certain phases ofthe operation. Comparable H /O ratios can be found by dividing any ofthe foregoing ratios by 3.2, e.g. with desirable ratios usually varyingfrom about 0.44 to 1.56 while recognizing that bootstrap operation maystill improve results even with ratios as high as 3 or 6 or more. Inview of the possibility of non-selective premature reaction of theoxygen with both CO and H it may be at times particularly desirable tostate concentrations in terms of the ratios of H to 0 as present whenthe rhodium catalyst is contacted; for such purpose any of the CO/Oratios described herein can be divided by 3.2.

It appears that the of CO in automotive exhaust increases rapidly andsubstantially linearly with increasing richness of the A/F ratio of therich side of stoichiometric. Moreover, this is accompanied by somedecrease in oxygen content. Thus the CO/O ratio ordinarily rises rapidlywith the richness of the feed. The addition of bootstrap air serves tomoderate this effect. With a larger amount of oxygen present, increasesin the CO do not have as great an effect upon the CO/O ratio. The resultis that the CO/O ratio stays at a lower level, and consequently Q alsoremains at low levels over a large range of air-to-fuel ratios.

Automobiles as now designed operate over a range of air-to-fuel ratios,rather than on a particular point, with a range of 1.0 A/F unit beingproposed for automobile models of the near future by engineers of aprominent automobile manufacturer. It is desirable to operate solely onthe rich side of stoichiometric, since it is not feasible to reducenitrogen oxides on the lean side of stoichiometric, and substantialoperation there will result in substantial discharge of nitrogen oxidesinto the atmosphere. It follows that operation of an automobile in thecontemplated manner will ordinarily involve some operation ranging atleast as far on the rich side of stoichiometric as 1.0 A A/F. l-loweveroperation at -l.0 A A/F, i.e. about 13.7 A/F, would not ordinarilyprovide sufficient oxygen to provent substantial ammonia production, butadditional oxygen would have to be added. Yet, if the rich edge of theA/F range were at l.0 A A/F, then the lean edge of a 1 unit wide A/Frange would be right at the stoichiometric point and no oxygen could beadded (assuming constant percentage addition of oxygen) withoutproviding an excess of oxygen at the lean edge of the operating rangeand thereby preventing reduction. Therefore in operating under richconditions for bootstrap operation, even the lean edge is ordinarily onthe rich side of stoichiometric by at least a small increment. In theforegoing discussion and unless otherwise specified herein, A/F ratiorefers to carburetion and does not take into account the amount ofbootstrap air added to the exhaust gases.

If bootstrap air is utilized, the amount added will depend upon theresults desired, for example an amount can be added sufficient to permit0 to be no higher than 20% at the most in the designated operatingrange, and preferably no higher than 10%.

As taught herein, the CO/O ratio is indicative of the ('2 levels to beobtained. If a particular width of A/F carburetion range is specified,along with error factor on the control of bootstrap air addition andwith known variation in CO and 0 with A/F ratios, it is possible todetermine the maximum CO/O ratio that can occur with a particularpercentage bootstrap air addition used to the best advantage under thespecified conditions.

7 Thus the percentage of bootstrap air can be selected to control theCO/O ratio at or below a desired level, eg

5, 4 or 3. The carburetion range can then be selected so that the leanedge is sufficiently on the rich side that the bootstrap air will placeit at or slightly on the rich side of stoichiometric. For example, withi% error in the bootstrap air control a 4.3% bootstrap air addition willindicate selection of a carburetion lean edge at about 0.7 A A/F.Similarly, 3 bootstrap air will call for about O.5 A A/F on the leanedge and 6% bootstrap air, for about O.9 A A/F on the lean edge.

Ordinarily, for reasons of fuel economy it will be desirable to have theA/F ratio no richer than necessary to insure against non-oxidizingconditions after addition of the bootstrap air, i.e. to have the amountof added air approach the maximum which can be added without making theconditions oxidizing on the lean edge of the A/F range. Of course someof the advantages of the bootstrap operation will be obtained even ifthe A/F range is richer than necessary. However, in most cases thepercent of bootstrap air will approach and be within 20% or so of themaximum which can be added without making the conditions oxidizing onthe lead edge, and the bootstrap air will be added in accord with theprocedures herein to avoid premature interaction. in general the propercombination of percent of bootstrap air and A/F range should be selectedto keep the CO/O ratio in a selected range, e.g. 1.5 to 4 or 5.

When bootstrap air is utilized, the percentage employed can vary overwide ranges with desirable effects, e.g. 0.5 to of the exhaust rate.Larger amounts of bootstrap air tend to have a greater effect. However,larger amounts also require a richer A/F ratio and therefore adverselyaffect fuel economics. Therefore the advantages must be balanced againstthe economic considerations. Thus a more practical range for operationis 2 to 10% bootstrap air, and amounts in the range of 3 to 6% aregenerally preferred. It will be understood that for bootstrap operationsall of the foregoing ranges will be used with A/F ratios which aresufficiently on the rich side of stoichiometric to avoid oxidizingconditions, in accordance with the teaching-herein.

In description of'the effects of bootstrap operation herein, particularattention has been devoted to worst case conditions, as customary inengineering design practice. However it should be recognized that the Qlevel will vary over the operating range, and that it may possibly befeasible to select the carburetion richness and bootstrap air rate toobtain desired average levels of 0, even though the maximum level of .Qbe higher than that ordinarily considered acceptable. Moreover, much ofthe discussion herein relates to continuous addition of air, forexample, addition of a constant percent of the exhaust stream. However,the addition rate could be varied, for example adding large amounts'ofair to exhaust during carburetion, and adding lesser or no amounts ofair when the carburetion is less rich, and such variants are to beconsidered within'the bootstrap operation described herein.

Further description of bootstrap operations with rhodium catalyst isfound in copending application Ser. No. 315,066 filed Dec. 17, 1972, andthe procedures and variants described there can be employed with theaddition procedures described and claimed here.

The foregoing describes the advantages' of bootstrap operations.However, in one aspect; inutilizing bootstrap air, it is desirable toadd it in'a particular manner toachieve the desired effect,-as describedand claimed in my copending application Ser. No. 324,286 filed Jan. 17,1973. The air should be added at a location and under conditions suchthat a suitable amount of oxygen is present and available for reactionwith the exhaust gas components upon contact with the catalyst.

' Oxygen is, of course, capable of reacting with both carbon monoxideand hydrogen.'The extent to which such reactions occur depends uponconcentration, temperature, presence or absence of catalyst, and otherfactors. It has been found that the reaction of bootstrap air withexhaust gases can be selectively directed toward reaction with hydrogen.If the bootstrap air, including its oxygen, is still present andwell-mixed with the exhaust gases upon contacting a rhodium catalyst,the oxygen can react with H in the gases in preference to CO in suchgases. This selectively is apparently dependent upon the rhodiumcatalyst. In contrast to this, if the oxygen and exhaust gases arepermitted to react prior to contacting the catalyst, the reaction isvirtually non-selective and substantial amounts of hydrogen may remainin the gases, along with CO, depending upon the overall stoichiometricrelationships.

Particular addition procedures described hereinare intended to lessenpremature, non-selective reaction of the bootstrap air with exhaust gascomponents. This reaction apparently occurs ina homogeneous phase. Anyreaction of the bootstrap oxygen with other exhaust components whichoccurs prior to contact with the rhodium/nickel catalyst will bereferred to herein as a premature reaction. It has been found thatexcessively high temperatures lead to premature reaction of the air withexhaust gas components. Thus exposure of the gas mixture to suchexcessively high temperatures should be avoided or minimized. The gasespresent in the engine manifold are, of course, at a very hightemperature. Therefore the bootstrap air should be admixed at a locationdownstream from the manifold. The time for possible premature reactionof the bootstrap air can be minimized by adding the air as close aspossible to the place of initial contact with the rhodium/nickelcatalyst. However, it is necessary to have the bootstrap air intermixedwith the exhaust gases in order to obtain the desired effect, andprovisions must be made for mixing prior to contact with the catalyst.It will be recognized that mixing the air and exhaust gases will requiresome finite time interval. In general the shorterthe time interval, theless chance for premature reaction. However, this must be consideredalong with the advantages of obtaining good mixing and practical meansfor doing so. Mixing means are known andavailable which provide therequired degree'of mixing in acceptable time intervals. It is preferredto add the bootstrap air immediately before contact with therhodium/nickel catalyst, and to rapidly mix the air with the exhaustgases prior tosuch contact. However, the air can be admixed earlier, andthe extent to which desirable results are obtained will be a function ofthe temperature, time, and other factors possibly initiating a prematurereaction.

It will be advantageous to employ rapid mixing means. It is desirablethat the streams of air and exhaust gases be intermingled, as byturbulent mixing, prior to contacting the catalysts. If there werestreamline flow with separate streams of the exhaust and air goingthrough the catalyst, the benefits of the bootstrap air additionwould'not be obtained.

The premature reaction of bootstrap air is undesirable as it isvirtually nonselective and may leave undesirable amounts of hydrogen inthe exhaust gases. It is therefore advantageous to avoid or minimizesuch reaction. Temperature has an important bearing upon the tendencytoward premature reaction. At temperature up to about 700C there isordinarily little tendency toward the homogeneous reaction, while thetendency toward such reaction becomes pronounced between 700 and 800C,although these values may vary with the surroundings, time, etc. It ispossible to rapidly admix air and exhaust gases in short time intervalsat temperatures up to 750C, or so without much reaction but this becomesmore difficult or uncertain as temperatures range on up to 800C orhigher. However it may be feasible to avoid reaction at these or evenhigher temperatures by optimization of jet or other mixing methods asapplied to the large flow involved in automotive exhaust. Thetemperatures referred to are oven measurement temperatures, and it willbe appreciated that actual gas or catalyst temperatures could rangeconsiderably higher due to exothermic reactions. Similar considerationscould, of course, obtain in an automobile exhaust gas stream. The actualtemperatures at the inlet to an NO, catalyst converter can, of course,vary with the location of the converter, modes of engine operation, andother factors. However, with the converter located in a post manifoldposition, the temperatures are expected to be in the 600 to 700C rangeordinarily, while possibly ranging up to temperatures over 800Coccasionally with particular driving modes. Thus it can be seen that thepresent invention can advantageously be employed. The addition of thebootstrap air downstream from the manifold avoids exposure of theair-exhaust mixture to the high manifold temperatures. The NO, converterinlet temperatures, although more moderate, can cause prematurereaction, and the addition procedures make it possible to shortenexposure of the admixture to such temperatures. This will be moreimportant for some inlet temperatures than for others, but is generallyadvantageous in view of the ranges of temperatures which can be expectedin the usual modes of engine operation.

The time factor is important in avoiding premature reaction but itsinfluence cannot readily be quantified. The tendency toward prematurereaction of the oxygen is influenced by temperature, gas composition,possible initiators, and other factors, as well as by time. Moreoveronce a homogeneous oxidation reaction is started, it may go very rapidlyto virtual completion. Despite this, utilizing a short mixing timeusually has beneficial results. Even if the temperature is so high thatsome homogeneous reaction will almost necessarily occur, the shortexposure may lessen the amount of such reaction. Moreover the exhaustgas stream is a flowing mixture of varying composition and varyingtemperature profiles. If a homogeneous reaction occurs at a particularstage and composition, it does not follow that such a reaction willnecessarily occur to the same extent when apparently similarcompositions and conditions again occur, as a different time factor orunknown factors may be involved. Thus shortening the exposure time canlessen the opportunity for such homogeneous reactions. The procedurestaught herein may not avoid premature reaction of the bootstrap airunder all temperature and other conditions which may be encountered.Nevertheless the procedures will give greatly improved results undersome of the conditions encountered, and will thereby improve the resultson an overall basis with respect to the average level of pollutantemissions. The selective oxidation utilized herein has advantages evenif practiced only on an intermittent or partial basis rather thancontinuously throughout the range of engine operations.

Rapid mixing can be employed in order to shorten the time during whichpremature reaction might occur. Even though a particular time intervalis suitable for some conditions, there may be advantage in using ashorter mixing time in the event other conditions are encountered. Thuswhile a time of 50 milliseconds or greater might be appropriate undersome conditions, there will generally be advantages in use of shortermixing times. For example, a time less than 20 milliseconds can beemployed, for example a time of 10 milliseconds or less is advantageous,and it will be advantageous to use a time of 5 milliseconds or suchlesser values as 2 milliseconds or 1 millisecond in so far as suitablemixing can be attained.

Various means can be employed to admix the bootstrap air rapidly intothe exhaust gas stream. The bootstrap air should be intimatelyintermingled with the exhaust gases, as by turbulent mixing, rather thanpresent as a separate stream when encountering the rhodium/- nickelcatalyst. For example, the type of combustible mixture of air andother'gases formed in a gas burner is ordinarily suitable. Various typesof turbulent mixing can be employed to obtain an intimate mixturerapidly. Jet mixers or injectors of various kinds can suitably beemployed. A two-jet mixer can be employed in which a jet of air impingeson a jet of the exhaust gases, in the manner used in oxyhydrogen torchesor for mixing or other combustible gases. Various types of injectors canbe employed in which a stream of air in an auxiliary pipe, jet, nozzleor tube, or orifice is injected into the exhaust stream in a main pipe.Mixing nozzles can be employed in which one gases passed through anarrow slot and then entrains gas from a concurrent stream. In such anozzle it will usually be advantageous to have the one gas, e.g. air,added with a velocity at least 2 or 3 times that of the main stream. Inan automobile exhaust stream, the air could be injected into the stream,say two pipe diameters upstream of the catalytic converter, with perhapstwo to four injection points around the circumference of the exhaustpipe being appropriate.

It is, of course, recognized that air can be added for purposes and atlocations other than those of primary interest herein, and the catalystsand bootstrap operations described herein can be utilized in conjunctionwith procedures in which air is added at cylinder ports or, otherwise inthe exhaust manifold, or in advance of an oxidation converter, orgenerally to use an NO, converter for oxidation during initial phases ofengine operation. In the event air is added anywhere upstream of the NO,converter, such addition should be taken into account in determining A/Fratio and the proper amount of bootstrap air to utilize.

In utilizing bootstrap operations herein, hydrogen may be substantiallyremoved from the exhaust gases over the rhodium containing catalyst. Toaccomplish this, oxygen can desirably be provided in an amount which isfrom of stoichiometric up to stoichiometric with oxidizable componentsat the lean edge of the A/F operating range.

The present catalyst utilizes a particular combination of components toachieve a desired effect. It will be understood, of course, that variousother active or inactive components can also be present so long as therequired components are present and in the proper arrangement. It may bedesirable to have various noble metals, e.g. platinum, palladium,ruthenium, present in small amounts in the nickel, and similarly it maybe desireable to employ promoters with the rhodium. It will beunderstood that the catalysts as described and defined herein includecatalysts containing such other components. The catalysts willordinarily consist essentially of the named components, but as in theusual interpretation of consist essentially, will remain open to thepresence of other components in amounts which do not have a basicdeleterious effect upon the properties of such catalyst.

The catalysts of the present invention have rhodium highly dispersed onan alumina surface, preferably an activated alumina surface. Transitionaluminas are suitable for use. By the term transition alumina is meantan alumina which is essentially alumina other than alphaalumina and alsoother than certain hydroxides of aluminum. Reference is made toTechnical Paper No. 10, second revision, from the Alcoa ResearchLaboratories. On page 9, various phases of alumina are enumerated. Thefollowing alumina phases are not generally components in the finishedcatalysts of the present invention.

Alpha-alumina The use of substantial quantities of the above citedphases is not generally made. Small amounts of such alumina phases maybe present, but are not the preferred starting materials for preparingthe catalysts of the present invention. A preferred support for therhodium in the present invention is composed predominantly of atransition alumina. Thus, a preferred alumina support for the rhodium inthe present invention may be composed predominantly of one or more ofthe alumina phases typified by various forms of gamma or eta alumina, ortheta, iota, chi, kappa aluminas, among others.

The form of the support may depend upon the specific application.However, it is likely that use of a monolithic carrier will beappropriate in view of the favorable heat-up properties of suchcarriers. Nevertheless the rhodium, alumina, nickel layered coatings canbe utilized whether or particles or on rigid, geometrical forms. Forexample, particle forms are exemplified by spheres, extrudats, rings,hollow cylinders, granules, or other shapes. Likewise, monolithicsupports, whatever their composition, can be coated with the specifiedmaterials.

Coatings can be applied in a variety of ways. A successful procedureinvolves use of dispersions of alumina powder. The dispersion isconveniently made by use of acidic aqueous suspensions created byagitation. Among the acids which can be used are acetic, hydrochloric ornitric. A simple, effective procedure for obtaining the amount ofcoating required utilizes, on a weight basis, 20.0 g alumina powder, 1.2g conc. HNO and 78.8 gH O. The alumina power is added to the acidifiedwater and then shaken vigorously to obtain a suspension. Suchsuspensions appear to be stable for at least two hours.

In the catalysts used herein the rhodium is preferably well-dispersed onthe surface of the alumina support without much penetration into thesurface of alumina particles. To obtain the desired effect from the veryminute amounts of rhodium involved, it is essential to have the rhodiumin position to contact the exhaust gases. It also appears advantageousthat the alumina, prior to rhodium depositions, be characterized by openpores with a minimum of small pores, as it appears that rhodiumdeposited in small pores is subject to occlusion so as to preventeffective catalytic contact. It has been found that precalcining thealumina-containing body prior to deposition of the rhodium produces acatalyst of better and more stable activity. It appears that thepre-calcination has the effect of reducing the size of or closing smallpores, thereby preventing penetration of the rhodium salts into suchpores and resulting in a greater concentration of the metals on theexposed surface of the alumina.

Monolithic supports can be used to advantage in the present invention.Such monolithic supports consist of unitary refractory or ceramicstructures characterized by a plurality of relatively thin-walledcellular channels passing from one surface to the opposite surface, thusproviding a large amount of geometrical surface area. The channels canbe of one or more of a variety of cross-sections selected from a varietyof shapes and sizes, each space being confined by ceramic wall. Crosssections of the support represent a repeating pattern which can bedescribed as lattice, corrugated, honeycomb, etc.

The dimensions of a suitable monolithic carrier for use in the presentinvention will depend on many factors including position of use in theexhaust train. Positions closer to the engine will favor more rapidheat-up as a consequence of higher exhaust gas temperatures. Generallyspeaking, when used in the post-manifold position, the volume of eachmonolith will be between about 15 and 80 in. and will have from about 8to about 14 corrugations per inch. Wall thicknesses will be from about0.005 to 0.015 inch thus creating an open area on the fact of themonolith of about 50 to The chemical composition of the monolithic orother support forms can consist of a-alurnina, mulite, cordierite,spodumene, Zircon, Alundum, magnesium silicate,

petalite, or combinations thereof, the refractory body being formed fromthese materials together with a suitable binder, such as clays, calciumcarbonate, calcium aluminate, magnesium aluminate or combinationsthereof. Generally, in the process of forming the rigid structureconsiderable porosity develops in the cell walls. For example, waterabsorptivities with B-spodumene monoliths may be 20 to 30% by weight.Inert support materials capable of accepting nickel deposition areadvantageous in providing a better base for fixing the nickel componentthan do activated alumina coatings. Inert materials, in contrast to theactivated, sorptive transition aluminas used to disperse rhodium inactive catalyst form, tend to have lower surface areas and activity andare classed as inert support materials.

The refractory bodies will have coatings of the designated materialsdeposited thereon. With the coating techniques outlined herein, thecoatings are fairly uniformly distributed through the channels, andinside the cell walls. Pores are generally blocked by the coatings.Coating thicknesses are on the order of one micron with a aluminacoating.

Alumina coatings on monoliths, when precalcined, will have surface areasin the range of about 50 to 200 m /g depending on precalcinationtemperature, when considered on the basis of alumina coating weightalone.

The coating techniques and requirements are equally applicable tonon-unitary, non-rigid substrates.

EXAMPLE 1 A catalyst was prepared by first depositing nickel on amonolith, then alumina, and finally rhodium. The monolith used was acordierite monolith having straight through channels, 200 channels persquare inch, with hydraulic diameters of about 0.06 inch, porosity ofabout 30%, and surface area of about 50 square inches per cubic inch.The monolith was impregnated by immersion in a nickel salt solutionhaving a concentration of 900 grams Ni (NO .6H O for 300 grams water.The monoliths were dipped up and down a number of times in the solutionand then immersed for to 25 minutes or so. Excess solution was removedin an air stream and the monoliths were then dried at 120C for l-2hours, followed by calcination in air at 550C for 2 hours. Theimpregnated monoliths were then coated with alumina from a aluminadispersion. The alumina dispersion was prepared by adding 40 grams ofalumina powder (Dispal M) to a solution of 1.2 grams concentrated nitricacid in 78.8 grams distilled water, and shaking for about 10 minutes.The monolith was completely immersed for upwards of 10 minutes, withoccasional dipping up and down. Excess solution was removed with an airstream and the coated monolith was dried at 120C for 2 to 4 hours andcalcined 5 hours at 600C. Rhodium was then applied by immersion in arhodium nitrate solution for fiteeen minutes with occasional clipping upand down. The solution had rhodium content of 9 micrograms per ml.Excess solution was removed with an air stream, and the impregnatedmonolith was dried at 120C for 2 /2 hours. Approximately 0.002 weightparts rhodium had been added per 100 weight parts original monolith. Thecatalytic monolith at this stage was subjected to the usual exhaust gastest conditions. It was then impregnated with additional rhodium, usinga 13.5 microgram/ml concentration and the same impregnation and dryingprocedure. The thus prepared dual catalyst had about 5 weight partsnickel, 3 weight parts alumina, and 0.005 weight parts rhodium per 100weight parts of monolith. The Rh/Ni catalyst with the nickel dispersedon a bare monolith, and rhodium dispersed on an alumina coating, waseffective for removal of NO, as shown from the results in Tables 2, 3and 4 below. Prior to testing, the catalyst was aged at 870C for 15hours in a gas mixture simulating a l.5 A A/F ratio. For comparison, 'asimilarly aged catalyst was employed having the same rhodium and aluminacontent but without the nickel undercoating. For convenience, the Rh/Nicatalyst is referred to as an Rh/Ni layered catalyst.

A gas stream was sampled to determine the extend to which oxidationmight occur at particular temperatures prior to contacting catalyst. Theusual test apparatus for measuring NO, catalyst effectiveness wasemployed, but without a catalyst and with sampling at the usual place ofinitial contact with catalyst. A simulated gas mixture was employed tocorrespond to a l.5 A A/F carburetion, with addition of 4.3% bootstrapair. In the procedure the gas mixture passed through an annularpreheater of about 15 ml volume at a rate of 4200ml/minute (STP). Thepreheater and catalyst were housed in a temperature controlled furnace.Results were as reported in Table 1.

Table l Remaining O NO, H 0 CO 700C Furnace 99 99 98 100 800C Furnace 9358 5 72 a catalyst temperature of 700C. Results are reported in Table 2.

Table 2 9 Without With Precombustion Precombustion Rh/Ni Lay. Cat. 5 l7Rh Cat. 8 52 It can be seen from the foregoing that the laminarrhodium/nickel catalyst is much better able to compensate forprecombustion of the bootstrap oxygen than is a rhodium only catalyst.It may be possible to minimize or substantially avoid precombustion byadding the bootstrap air shortly before catalyst contact and limitingexposure to high temperatures as further taught in my copendingapplication Ser. No. 324,286 filed 1 /17/73. However some hightemperatures and precombustion may still occur occasionally in somemodes of engine operation and it is advantageous to provide an abilityto compensate therefor.

Another advantage of the combined rhodium and nickel catalyst isimproved ammonia selectivity, as shown by a lower Q at the rich edge ofoperating conditions, e.g. at l .7 A A/F with a 4.3% bootstrap airaddition. Comparative results for this carburetion extreme are shown inTable 3, at the nominal bootstrap air rate, along with results at suchcarburetion extreme and the worst case bootstrap air rate, 3.5%,obtainable with i 20% error on the bootstrap air-addition.

Table 3 A/F, added air Rh Cat. Rh/Ni Lay. Cat.

Al.7, 4.3% l4 9 Al.7, 3.5% 26 17 The better results at 4.3% bootstrapair also demonstrate the effect of bootstrap air and the advantage ofadding it in preferred amounts with respect to the selected A/F ratio. 7

Another advantage of the rhodium/nickel catalyst is enhanced cold-startperformance as measured by the NOXIM test with a 500C furnace, Al.5 A/Fand 4.3% bootstrap air, and 17% secondary air during the first 2 minutesfrom cold start. In the NOXIM test the hydrocarbon (HC) and carbonmonoxide (CO) data represent the integrated performance of the NO,catalyst acting as an oxidizing catalyst during the 2 minute cold start,while the Q value represents the integrated performance during minutes 3to 5 following the cold start, and represents the second cycle of agovernment established test (CVS test). Results are:

Table 4 Remaining Rh Cat. Rh/Ni Lay. Cat.

HC 9l 84 CO 36 27 Q 19 14 EXAMPLE 2 buretion, and the results, in termsof approximate Q values, are reported below with respect to variance inthe CO/O ratio in the test gas. The CO/H ratio was set at 3.2.

Table 6 Oven Temp. Remaining (C) 9 NO,

Rh Cat.

Slow Mix 750 26 14 Fast Mix 800 23 13 Rh/Ni Lay. Cat. A y Fast Mix 700.3 Fast Mix 800 5 0 The layered Rh/Ni catalyst employed in Example 3 wasprepared as follows. Nickel was impregnated on a monolith by dipping themonolith for two minutes into a solution containing a concentration of0.236 gram nickel per cc. (as Ni(NO The channels of the monolith werecleared of excess liquid and the monolith was dried at 120C, andcalcined inair at 550C. The nickel-coated monolith was then coated bydipping into a 20% alumina dispersion for 2 minutes, cleared of excessliquid, dried at 120C, and calcined at 600C. The coated monolith wasthen impregnated with rhodium by dipping into-a rhodium nitrate solutioncontaining 0.000125 gram Rh/ml. Excess solution was removed and thecoated monolith was dried at 1 20C. The catalytically-coated monolithwas then aged for 16.5 hours at 870C in a simulated exhaust gas stream(l.5 A A/F). The final composition was about 15.74 weight parts nickel,4.2 weight parts alumina and an outer'impregnate of 0.02 weightparts'rhodium, per 100 weight parts monolith.

EXAMPLE 4 Gas mixtures were employed as in Example 1 to simulate aprecombusted mixture of exhaust gas and air, and a mixture withoutprecombustion. The gas mix- Table 5 tures were used to determine theeffectiveness of catalysts with results as reported in Table 7. The sameCO/O 0 Rh/Ni catalyst was used as in Example 3.

3.0 6.45 Table 7 3.6 7.35 4.15 9.45 4.6 l2.0 Remaining 5.45 16.5 o No, H

Rh/Ni Lay. Cat. There is advantage in operating at the lower CO/O rap 218 30 tios, such as below 5 or even lower, down to the stoigi gggiz 5 2 0chiometric point. By employing a bootstrap operation 52 30 in which richcarburetion is combined with addition of w/o pre-combust. 9 2 0 properamounts of supplemental air, it is possible to maintain low CO/O 'rati0sover broader ranges of A/F ratios, and such bootstrap operation isordinarily advantageous with the catalysts of the present invention.

EXAMPLE 3 It is apparent that the premature reaction of oxygen has adeleterious effect upon the catalyst performance, although the Rh/Nilayered catalyst performance is still fair. The poor performance of thepre-combusted mixture also appears related to the continued presence ofsubstantial amounts of hydrogen.

EXAMPLE A nickel on monolith catalyst was prepared by ordinarydeposition procedures, such as those described in Example 1. Thenickel-coated monolith was calcined at elevated temperature. Smallamounts of platinum and rhodium were also deposited from salt solutionto be included in the nickel coating. The thus coated monolith had 10.11weight parts nickel, 0.005 weight parts platinum, and 0.005 weight partsrhodium per 100 weight parts of the monolith. The rhodium and platinumwere included as promoters to enhance the reducibility of nickel oxidesor other oxidized forms of the nickel, but are not essential to theeffectiveness of the nickel component. A cordierite monolith asdescribed in Example 1 was employed. The coated monolith was then coatedwith alumina by dipping in a aqueous dispersion of alumina powder(Dispal M). Two successive dippings were used, with drying at 120Cfollowed by calcining at 600C. The total amount of alumina present aftercalcination was 4.31 weight parts per 100 weight parts of the monolith.The thus coated monolith was then impregnated with rhodium by immersionin a rhodium nitrate solution having a concentration of 9 microgramsrhodium per ml., followed by drying at 120C. The amount of rhodium thusprovided was 0.0026 weight parts per 100 weight parts monolith. The thusprepared Rh/Ni layered catalyst was tested in the state as made, andafter hydrothermal aging (HTA) at an inlet temperature of 870C, for 22hours. For comparison, a 0.0025% rhodium on alumina-coated monolith(B-spodumene) was employed. Also a sequential rhodium and nickelcatalyst was used for comparison, in which the described rhodiumcatalyst was followed by a nickel catalyst on an uncoated monolith. Forfurther comparison, an additional catalyst, designated as a co-catalystis included, in which both rhodium and nickel were deposited on analumina coated monolith. The co-catalyst had 3 weight parts alumina, 5weight parts nickel, and 0.002 weight pans rhodium per 100 weight partsmonolith. Results were as follows, employing a l .5 A A/F feed with 4.3%bootstrap air:

The rhodium/nickel layered catalyst gave good results even at fairlyhigh inlet temperatures, and also gave a surprising improvement inperformance upon aging. It will be noted that the performance from theadditive effect of the nickel and rhodium in the sequential arrangementis'inferior to that of the Rh/Ni laminar catalyst, even though thelatter catalyst occupies only onehalf the volume of the combinedsystems. Several of the catalysts were then tested in a NOXIM test at anoven temperature of 500C, with results as follows:

Table 9 Remaining H CO Lay. Rh/Ni Fresh 18 79 26 HTA 13 35 HT A 18 96 34Co-cat. Rh/Ni Fresh 25 77 21 HTA 19 97 34 From tables 8 and 9 it can beseen that the layered Rh/Ni structure compares favorably ineffectiveness with the co-catalyst structure.

EXAMPLE 6 Accelerated aging tests were conducted with the layeredrhodium/nickel catalysts described herein to determine resistance toaging under conditions possibly occurring in use. In the test thecatalyst was subjected at 750C to a gas stream corresponding to a richA/F ratio with the proper amount of bootstrap air addition, with anadditional 17% air pulse (1 minute on, 1 minute off) over the course ofa 1-hour test period. This provides a cycle of high temperatureoxidizing and reducing conditions such as might occur in certain vehicledriving modes. The layered catalysts exhibited little change in activityover the course of a fifteen cycle test. This is a very severe test, andequivalently severe aging might occur only after prolonged vehicleoperation. A rhodium and nickel on alumina co-catalyst, as describedherein, also showed activity retention in the aging test. However, itwas observed that the cocatalyst became fluffy with spalling of theactives coating, indicating that it would eventually lose activitybecause of physical degradation, and loss of catalytically activecomponents. Apparently some latice size changes in theoxidation-reduction cause physical degradation.

For much of the catalyst evaluation discussed herein, a special CRANOXtesting system was used. CRANOX (Catalytic Removal of Automotive NO,) isa fully automatic catalyst testing system controlled by a specialpurposedigital controller. The control system is capable of testing a singlecatalyst at six different feed compositions (varying the air/fuel ratio)and at up to 256 different temperature levels. All pertinent data areacquired by the system, processed through a digital integrator andteletypewriter which generates a paper tape record of the run. This tapeis processed by off-line computer which generates tables and graphs ofcatalyst performance parameters.

The CRANOX analytical system uses a dual-bed technique to determine thesum of ammonia and unconverted NO, leaving the reactor: a metered sampleof the reactor product is diluted with a metered quantity of air and themixture passed over a platinum-onalumina oxidation catalyst. Theeffluent is then sent to an electrochemical transducer which measuresthe sum of NO and N0 an electrochemical cell operating on the principleof a fuel cell (Dynasciences NX-l30 analyzer) being used for thispurpose. Oxidation reactor conditions were found that resulted inquantitative conversion of ammonia to NO,. The determination of the sumof ammonia plus unconverted NO, is an especially important measure ofthe effectiveness of an automotive NO, catalyst as it measures theworst-case performance that might be achieved in an actual dual-bedconverter that would oxidize any ammonia formed in the NO converter backto NO, in the oxidation converter.

The CRANOX feed-gas system generates the six feeds by synthetic dynamicblending of eight components to achieve a simulated automotive exhaustthat is very close to an actual exhaust in which the air/fuel ratio isvaried by carburetion changes. The flow rate over the test catalyst canbe held constant, for example at 4200 ml/min (STP) for all catalystfeeds. With a normal catalyst sample of 3 ml. this results in a spacevelocity of 80,000 hr A space velocity of 90,000 hr is also oftenemployed. The catalyst is contained in a quartz tube 16 mm. [.D. whichis in turn contained in a quartz test tube 25 mm OD. The flow pattern isarranged such that the incoming feed is preheated by flow through theannular space between the two tubes. This also results in near-adiabeticoperation of the inner tube containing the catalyst. Both tubes arehoused in a furnace consisting of a 1% inch stainless steel pipemaintained at a controlled temperature by radiant heat transfer fromelectrically heated coils surrounding the pipe. In the tests herein, aspecified boot strap air rate was generally employed with a specified AA/F. Such factors correspond to particular concentrations of exhaust gascomponents. For example, a 1.0 to A A/F range, with no air addition,corre sponds to about 3.28% CO and 0.12 0 at the rich edge, and 0.7% COand 0.51 0 at the lean edge. A l .7 to 0.7 A A/F range after addition of4.3% bootstrap a'ir corresponds to 4.04% CO and 0.98% 0 at the richedge, and 1.82% CO and 1.18% 0 at the lean edge.

EXAMPLE 7 Table 10 Q at A A/F Ni/Rh Cat. l.7 A l.5 A l.0 A

Fresh 16 12 HTA 27 23 8 Ni/Alzoa/Rh Cat.

Fresh 12 7 2 HTA 10 6 2 While the catalyst will work without the aluminalayer, the advantage of the alumina layer is apparent. The catalystswere then tested in a NOXlM test at 500C.

Table l 1 Ni/Rh Cat. 9 HC CO Fresh 24 90 36 HTA 47 98 63 A layeredcatalyst was prepared containing 6 weight parts nickel, 1.7 weight partsalumina, and 0.004 weight parts Rh per 100 weight parts monolith,thematerials being deposited in the stated order. The alumina coatedmonolith was calcined at 600C prior to deposition of the rhodium. Asimilar catalyst containing the 1 same nickel and. rhodium contents and3.6 weight parts alumina, was calcined at 870C after the aluminadeposition. The catalysts were dried at 120C,- hydrothermally aged andtested at 600C with results as follows:

Table 12 n at A A/F Catalyst l.7 l.5

600C calcine 17 1 1 870C calcine l0 7 The catalysts were then tested ina NOXIM test at Table 13 remaining Catalyst (2 HC CO 600C calcine 18 8931 870C calcine 16 91 37 0 It can be seen that the calcination at highertemperature made the catalyst considerably more effective in reducingNO, emissions, presumably because 'of some reduction of pore size orsurface area of the alumina coating. Moreover the NO, emission resultswere essentially equivalent to those obtained on the same type oflayered catalyst prepared with an 870 calcination but with twice therhodium content, 0.008 weight part. Thus the precalcination is alsoadvantageous in making it feasible to reduce the content of the scarcerhodium component. The results indicate it is desireable to calcine thealumina coated material at temperatures above 600C, with a temperatureof about 870C or higher being suitable. The changes leading to theimproved results are presumably a gradual phenomenon, but temperaturesof 700 to 750C or above 700C would probably be sufficient to obtain mostof the benefits of precalcination, depending somewhat on the heatingtime. The heating at 870C in a muffle furnace for 3 /2 hours isaccompanied'by a change in appearance in that the material becomes amuch brighter green, compared to the grey green of material calcinedonly at 600C. This apparently indicates more nickel aluminate formation,and the nickel aluminate appearcination to achieve improved activity,although I changes in surface area may be as or more significant thanchanges in chemical form.

The transition aluminas employed herein as the alumina coating includeboth gel-derived and crystallinederived aluminas, although it isindicated that somewhat better performance can be obtained with the gelderived aluminas.

As described herein, the rhodium-nickel layered catalyst has advantagesfor removal of nitrogen oxides from exhaust gases, particularly in thehigher ranges of temperatures encountered. The usefulness of thecatalyst is also enhanced by the use of bootstrap operations asdescribed herein, especially under conditions to avoid prematurereaction of the oxygen. However, regardless of how or where air is addedor the amount of premature reaction, good results can be achieved in abootstrap operation if the oxygen is in proper ratio to the carbonmonoxide and hydrogen at the time the catalyst is contacted. Of coursethere will be an economic penalty if excessively rich carburetion isemployed to allow for addition of high amounts of air and prematurereaction. It will therefore generally be preferable to operate underconditions to avoid or substantially minimize premature reaction.

EXAMPLE 9 A good rhodium/alumina/nickel on monolith catalyst, having anQ of 6 upon testing at 600C and l.5 A A/F, was tested to determine theextent to which activity was recovered after oxidation. The catalyst wasaged in air at 800C for one week. The initial value was then 80, but thevalue declined to 55 after exposure to the -Al.5 A/F stream for 6minutes. After 10 minutes exposure at 870, the value was 9, and thecatalyst then gave a test value of 7 at 600C.

I claim:

1. A catalyst comprising nickel disposed on an inert support and analumina coating thereover upon which coating an amount of rhodium nogreater than 0.1 weight part per 100 weight parts inert support isdispersed, with the nickel and rhodium being oxides or in elementalform.

2. The catalyst of claim 1 in which the amount of rhodium is less than0.01 weight parts per 100 weight parts inert support.

3. The catalyst of claim 1 in which the amount of nickel is in the rangeof 0.5 to 50 weight parts and the amount of alumina in the range of 0.1to 20 weight parts, each per weight parts inert support.

4. The catalyst of claim 3 in which the amount of rhodium is 0.0005 to0.05 weight parts.

5. The catalyst of claim 1 in which there are at least 5 weight partsnickel, 2 to 6 weight parts alumina, and the amount of nickel is atleast twice that of alumina.

6. The catalyst of claim 1 in which the support is a monolith.

7. The catalyst of claim 1 in which the support is a cordieritemonolith.

8. The catalyst of claim 1 in which nickel is deposited on a monolith,followed by an alumina coating, and then rhodium is deposited.

9. A catalyst comprising nickel fixed on an inert support and aluminadeposited as a coating over the nickel with rhodium dispersed upon thealumina in an amount of 0.0005 to 0.05 weight parts per 100 weight partsinert support, with the nickel and rhodium being oxides or in elementalform.

10. The method of preparing a catalyst by impregnating an inert supportwith a nickel salt solution, drying and calcining; then coating aluminathereon from an alumina suspension, drying and calcining; and thencalcining the coated support with a rhodium salt solution, drying, andcalcining to decompose the rhodium salt.

11. The process of claim 10 in which the coating of alumina is followedby calcining at temperature above 700C prior to impregnating with therhodium salt solution.

12. The process of claim 10 in which the coating of alumina is followedby calcining at 870C to the appearance of nickel aluminate, becoming amuch brighter green than that characteristic of material calcined at600C, prior to impregnating with the rhodium salt solution.

13. The process of claim 10 in which the drying and calcining steps areat 1 10 to 200C and 400 to 600C, respectively and the amounts are suchas to give 0.5 to 50 weight parts nickel, 0.1 to 20 weight partsalumina, and 0.0005 to 0.05 weight parts rhodium, per 100 weight partsinert support.

1. A CATALYST COMPRISING NICKEL DISPOSED ON AN INERT SUPPORT AND INALUMINA COATING THEREOVER UPON WHICH COATING AN AMOUNT OF RHODIUM NOGREATER THAN 0.1 WEIGHT PART PER 100 WEIGHT PARTS INERT SUPPORT ISDISPERSED WITH THE NICKEL AND RHODIUM BEING OXIDES OR IN ELEMENTAL FORM.2. The catalyst of claim 1 in which the amount of rhodium is less than0.01 weight parts per 100 weight parts inert support.
 3. The catalyst ofclaim 1 in which the amount of nickel is in the range of 0.5 to 50weight parts and the amount of alumina in the range of 0.1 to 20 weightparts, each per 100 weight parts inert support.
 4. The catalyst of claim3 in which the amount of rhodium is 0.0005 to 0.05 weight parts.
 5. Thecatalyst of claim 1 in which there are at least 5 weight parts nickel, 2to 6 weight parts alumina, and the amount of nickel is at least twicethat of alumina.
 6. The catalyst of claim 1 in which the support is amonolith.
 7. The cataLyst of claim 1 in which the support is acordierite monolith.
 8. The catalyst of claim 1 in which nickel isdeposited on a monolith, followed by an alumina coating, and thenrhodium is deposited.
 9. A catalyst comprising nickel fixed on an inertsupport and alumina deposited as a coating over the nickel with rhodiumdispersed upon the alumina in an amount of 0.0005 to 0.05 weight partsper 100 weight parts inert support, with the nickel and rhodium beingoxides or in elemental form.
 10. The method of preparing a catalyst byimpregnating an inert support with a nickel salt solution, drying andcalcining; then coating alumina thereon from an alumina suspension,drying and calcining; and then calcining the coated support with arhodium salt solution, drying, and calcining to decompose the rhodiumsalt.
 11. The process of claim 10 in which the coating of alumina isfollowed by calcining at temperature above 700*C prior to impregnatingwith the rhodium salt solution.
 12. The process of claim 10 in which thecoating of alumina is followed by calcining at 870*C to the appearanceof nickel aluminate, becoming a much brighter green than thatcharacteristic of material calcined at 600*C, prior to impregnating withthe rhodium salt solution.
 13. The process of claim 10 in which thedrying and calcining steps are at 110* to 200*C and 400* to 600*C,respectively and the amounts are such as to give 0.5 to 50 weight partsnickel, 0.1 to 20 weight parts alumina, and 0.0005 to 0.05 weight partsrhodium, per 100 weight parts inert support.